1. Field of the Invention
The present invention relates to on-screen displays, particularly such displays as are commonly used on the picture screen when a television tuner is in the auto-search mode.
2. Description of the Related Art
An On-screen display (OSD) typically comprises alphanumeric and/or picture characters to provide information to the user, for example, regarding channel number, programming choices, and prompting statements. Typically, the OSD signals are supplied to the luminance/chrominance processing part of the television receiver to be displayed on the screen superimposed on any picture signal received by the tuner. To ensure that the OSD always appears in the same position on the screen, it is synchronized with the horizontal and vertical deflection timing pulses which are separated in the receiver from the received television signal.
A problem arises when an OSD is required at a time when no picture signal is being received, for example, during tuning, or if the picture signal is weak. Then either no timing pulses are present or the timing pulses are very noisy because of residual harmonics which appear as false video synchronizing signals at the output of the picture intermediate frequency demodulator (further referred to as “PIF demodulator”) which demodulates the intermediate frequency supplied by a tuner into a baseband video signal. This tends to produce disturbances and distortion in the OSD, including jagged edges and vertical bounce because the position of the OSD characters will tend to vary from line to line and from field to field. This is evidently undesirable. The problem is particularly acute during an auto search tuning mode of a television receiver.
It has been proposed to use a substitute synchronizing signal in circumstances when no suitable external signal is being received. This increases the cost of the receiver. Alternatively, U.S. Pat. No. 4,677,484 proposes a dedicated deflection signal source and control means to switch the source from an operating mode, in which it is synchronized with a received synchronizing signal, to a mode in which it is free running, when the incoming signal is determined to have an invalid or unsuitable synchronizing signal. This again requires expensive additional circuitry.